This invention relates to a failure detecting apparatus for a dual valve provided in a pneumatic circuit for supplying a pneumatic pressure to a clutch and brake adapted to actuate and control movable parts of machine tools or the like.
Movable parts of conventional machine tools etc, in particular, slide driving mechanisms of press machines are provided with a clutch and brake to which supply of compressed air is made or interrupted thereby to drive or stop the slide.
Further, solenoid valves are provided in the circuit for supplying pneumatic pressure to the clutch and brake.
Therefore, if the solenoid valve goes wrong, the slide cannot be stopped at its top dead centre and will move downwards so that it will strike twice and so the operation itself is very dangerous.
As a countermeasure for this, a dual valve comprising a pair of solenoid valves integrally arranged in parallel is provided in a pneumatic pressure supply circuit so that even if either one of the solenoid valves goes wrong, another solenoid valve is actuated so as to prevent the occurrence of two times of striking of the slide.
However, since it is dangerous to continue the operation as one of the solenoid valves remains in trouble, detection of the failure of one of the solenoid valves is required.
For this purpose, there has heretofore been employed a failure detecting apparatus adapted to detect the failure of one of the solenoid valves by detecting differential pressure; however, such construction is disadvantageous in that if the failure detecting apparatus of differential pressure detection type goes wrong, failure of the solenoid valve cannot be detected so that provision of a separate test circuit is required, and at the time of start and periodically the failure detecting apparatus of differential pressure detection type needs to be inspected and so it is complicated in construction and requires troublesome operation with low reliability in failure detection.